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LT3597

3597f

OPERATION

The LT3597 uses a constant frequency, internally com-

pensated peak current mode control scheme. Operation

is best understood by referring to the Block Diagram in

Figure 1.

Enable and undervoltage lockout (UVLO) are both con-

trolled by a single pin. If the pin falls below 1.51V, an

accurate comparator turns off the LED drivers and buck

regulators. If the pin continues to fall to less than 0.4V,

the part enters shutdown and consumes less than 2µA.

The LT3597 contains three constant current sink LED

drivers. Each of the three LED strings is powered from

a dedicated adaptive buck converter in order to achieve

maximum efficiency. The frequency of the buck regulators

is programmed from 200kHz to 1MHz using an external

resistor. The frequency can also be synchronized to an

external clock using the SYNC pin.

Internal buck compensation and soft-start requires few

external components and permits simple board layout. A

high-side switch current limit protects the internal switch

during its on time, while a low side current limit prevents

the switch from turning on in the case of excessive off

phase current.

Step-Down Adaptive Control

Adaptive control of the output voltages achieves superior

system efficiency. When a given channel’s PWM pin is

low, the respective buck regulator output will go to a

programmable high output voltage. In this case the buck

will enter into a pulse-skipping mode since there is no load

connected. This guarantees that the buck output voltage

is high enough to immediately supply the LED current

once the string is reactivated. As soon as PWM goes

high, the output voltage of the buck will drop until there

is 1V across the LED current sink. This scheme ensures

the best efficiency for each LED channel. Since each LED

string is independently driven from a separate buck chan-

nel, efficiency is optimized for all three strings even if the

number of LEDs is mismatched between the channels.

AnotherbenefitofthisregulationmethodisthattheLT3597

startsupwith10,000:1dimmingevenifthePWM1-3pulse

width is 1µs. Since VOUT starts up even if PWM1-3 is low,

the part achieves high dimming ratios with narrow pulse

widths within a couple of PWM1-3 clock cycles.

LED Current

Each LED string current can be individually programmed

up to a maximum of 100mA with a 2% matching accuracy

between the strings. An external resistor on the ISET1-3 pin

programs the max current for that string. The CTRL1-3

pin can be used for analog dimming. Digital PWM can be

programmed using the PWM1-3 pin. A dimming ratio of

10,000:1 can be achieved at a frequency of 100Hz.

Fault Protection and Reporting

The LT3597 protects against both open LED and shorted

LED conditions. If the LED1-3 pin voltage exceeds 12V

while the LED string is sinking current, or if the LED1-3

pin voltage is within 1.25V of VOUT1-3 pin voltage, the

channel is disabled until the fault is removed. If the LED

string opens, the LT3597 will limit the output to the voltage

set by the FB resistor divider.

The LT3597 reports a fault on the FAULT pin if any of the

LEDstringsisopenorshorted.LEDfaultsareonlyreported

if the respective string PWM signal is high. A fault is also

reported if the internal die temperature reaches the TSET

programmed derating limit.

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